The present invention relates to an improved method and apparatus for preparing uniformly-sized particles.
The techniques of producing fine particles has been important for the industries which manufacture pigments, abrasives, photographic films, magnet recording tapes, powder metallurgical wares, electronic ceramic materials, and electric storage batteries. Among the characteristic properties of fine particles, particle sizes and their distribution affect the quality of the final products when the particles are used as a raw material. For example, silver bromide particles having a wide distribution of particle sizes cause an undesirable phenomenon known as pepper fog when they are used as a photographic material. When gamma ferric oxide particles which are used for magnetic recordings have a wide distribution of particle sizes, they cause an increasing ratio of noise intensity to signal one recorded on the tape. This leads to a poor recording characteristic for weak signals.
On the other hand, oxide particles used as a raw material for electronic ceramic or thick film resistors leads to high reproducibility, reliability, and potentiality of the components or devices when they possess a uniform particle size. As mentioned above, a narrow distribution of the particle size plays an important role for various material technologies.
There have been several attempts to prepare fine particles having a narrow particle size distribution, i.e., a substantially uniform size throughout. Exemplary of the improvements in silver halide precipitation techniques are U.S. Pat. No. 2,996,287, U.S. Pat. No. 3,415,650, and U.S. Pat. No. 3,782,954. Accordingly, it is an object of the present invention to overcome the above-mentioned disadvantages and to provide an improved method for preparing uniformly-sized fine particles.
Another object of the present invention is to provide an improved apparatus for preparing uniformly-sized fine particles.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Pursuant to the present invention, uniformly-sized particles can be produced in the following manner: a first reactant solution, for example, a silver salt, is poured into a cup disposed within a reaction vessel, said cup rotating at a high speed. The reactant solution introduced into the cup is ejected from the cup by the centrifugal force of the rotating cup, forming uniformly small droplets. A second reactant solution, for example, sodium hydroxide or potassium hydroxide, is uniformly introduced along the inside wall of the cylindrical reaction vessel. As a result, the above-mentioned centrifugal action small droplets of the first reactant solution are directed against the falling flow of the second solution and contact the second reactant solution flowing along the inside wall of the cylindrical reaction vessel causing a chemical reaction to occur in the form of precipitated particles, for example, silver oxide particles. The particles thus produced possess a substantially uniform particle size.